1. Field of the invention
This invention relates to a fuel cell stack and a method of pressing its elements together wherein the fuel cell stack includes a plurality of fuel cell separators, each disposed between fuel cells and having a space therein into which a pressurized fluid is supplied to apply the pressure developed by an internal pressure of the space to the neighboring fuel cells.
2. Description of the Related Art
Developments in fuel cells ("FC"s) are remarkable today. An FC generates electric power from a reaction gas. FIG. 1 shows an internal manifold type FC as an example of conventional FCs. The structure of this type FC will be described hereafter.
An FC stack ("S") is constructed by stacking a number of unit fuel cells ("TC"s) on one another and mounting end plates ("EP"s) to both ends of the stack of the TCs. The TC has a power generating unit ("H") and a pair of holding units ("K"s) disposed on both sides of the H. The H comprises an electrolyte layer and a pair of catalytic electrodes ("CA"s) attached on both sides of the electrolyte layer. A pair of collectors ("CO"s) are positioned between the H in the center of the TC and the Ks on both sides of the H.
Then, the EPs are pressed together by compression means such as bolts B with coil springs C. That is, the conventional method of pressing the FC stack together has been such that the H and the Ks of each TC are simultaneously clamped by compression means such as the bolts B.
In the above S, one of the EPs acts as a pressure generating member ("MH"), in which a recess ("HB"), covered with a membrane ("L"), is defined and a balloon is provided in the HB so as to be inflated to fill the HB. Then, H.sub.2 or O.sub.2 as a reaction gas ("G") for the FC is introduced into the MH through a supply line ("KP") connected to the MH through the EP, and inflates the balloon in the MH. As a result, the L is flexed and pressed against the neighboring TC, applying a compression force thereto.
Stacking the TC on one another while applying a pressure to each TC, as described above, enables the FC to avoid variation in contact resistance between the electrode catalysts and collectors, between the H and Ks, and between the TCs, to assure a constant output voltage therefrom.
There have been, however, problems in the conventional method of stacking FC, as follows.
(A) In the method of stacking the FC as shown in FIG. 1, a recess is defined on an inner surface of the K in which respective one of the CAs and COs are held. It is therefore necessary to accurately match the depth of the recess of respective Ks with the thicknesses of the CA and CO. That is, each TC must be fabricated with high accuracy.
(B) In the above method of stacking the FC, the compression bolts B have twofold tasks: one is to seal passages for reaction gas internally defined in the respective members by pressing the respective members against each other, and the other is to press the CAs and COs in respective TCs. However, required pressure for sealing the passages and pressing the CAs and COs are generally different, because the purpose and use of them are different from each other. Therefore, when the bolts B are tightened under a suitable pressure to press the CAs and. COs, there is a danger of inappropriate sealing of the passages, and vise versa.
(C) In the conventional method described above, the compression force is applied on both ends or only one end of the stack S to press the constitutive members. Thus, as the number of the TC to be stacked increases, the number of members having force relaxing nature (such as sealing members) also increases. As a result, the compression force applied to the S is not sufficiently transferred to each of the TCs, due to partly deformation of respective members, causing insufficient pressing together of the TCs.
In order to overcome the inconvenience raised in (C), it has been necessary to adopt either one of applying a larger force to press together the constitutive members, or restricting the number of the TCs in one stack S. However, when the former is selected, the use of fragile materials such as carbon, plastic, ceramic, etc. in each TC must be restricted.
When the latter is selected, it is necessary to series-connect several Ss each having a restricted number of TCs therein to obtain a desired voltage. This makes piping for the reaction gas, or arrangement of auxiliary instruments, etc. complex, and increases the space to be occupied by the FC. Further, an integrated control of the series-connected Ss is not necessarily easy.
(D) In the conventional FC, it has often been seen that one cooling plate is placed for a series of several TCs in one S. In this structure, the temperature of a series of TCs placed between a pair of cooling plates is maintained constant, but that constant temperature may be different from the temperatures of other series of TCs. This temperature difference can cause an insufficient cooling of the TCs in an operation at a high current density.
In addition to the above, a method of compressing the S has been proposed by U.S. Pat. No. 4,317,864. In this method, contact members are placed between respective TCs constituting an S, each contact member having a chamber defined therein. A pressurized fluid to be fed to the TCs is introduced into the chambers so that the contact members are expanded to press the CAs. Thus, the COs are brought into close contact with their corresponding CAs. Therefore, if the contact members are provided for every TCs, a pressure different from the pressure used for sealing the fluid passages can be applied to each CA. In stacking a plurality of TCs on one another, a uniform pressure can be applied to the respective TCs.
However, since the COs employed in each TC are thin, it is difficult to apply a uniform pressure to the COs through the expanding contact members. Further, the pressure applied to each CO is low, because the pressure of the pressurized fluid has been exhausted in expanding each contact member. That is, the efficiency in transferring pressure is poor. As a result, the contact resistance between the CAs and COs cannot be lowered sufficiently and a desired output voltage cannot be attained.